How to Build a Dinosaur (4 page)

That was a difficult summer for Montana in other ways. Ted Kaczynski was found living in a cabin in Lincoln, on the western side of the Rockies. Kaczynski, the notorious Unabomber, was bigger news than the Freemen. He had sent bombs through the mail to people he thought were responsible for the ruin of modern society by technology. Over the course of about two decades he killed three people and wounded twenty-two. He was eventually identified by his own brother and he turned out to be an academic, with a Ph.D. in mathematics, who had gradually detached himself from society and embarked on a violent crusade.

It’s an old rhetorical flourish to tie politics to the land, and often false. The Freemen had no support from the population around Jordan, partly because they didn’t do any honest work. But it is true that eastern Montana is extreme even in a state given to extremes, in landscapes, weather, and history. And as life gets easier in other parts of the country, it just seems to get harder to make a go of it in Garfield County. The land can be harsh to the point of desolation. In the summer, dry mudstone flats bake in 120-degree heat and drinking a gallon or two of water a day to keep hydrated becomes a matter of survival. In the winter the wind rages at 40 below zero. The end of nature may have arrived in principle, but in a place where cell phone signals often disappear, the ancient hazards have not lost their power.

The Missouri River is the county’s northern border, in the form of Fort Peck Lake, 134 miles long, rich with fish and often shrunken by drought. The lake is a product of the Depression-era Fort Peck Dam, built from 1933 to 1937 to provide power and jobs. Ten thousand people worked on the dam, just over the line in McCone County. Since the dam was finished, nothing else has brought people to this part of Montana in those numbers.

The geological past seems to dominate the human story here the way the weather can overwhelm philosophical musings about the planet. The earliest ranches here are only a few generations old. The hold of people on the badlands feels tenuous. The fossil hunters for the great New York and Washington museums, who arrived at about the same time as settlers, struck it rich, so to speak, filling the halls of natural history museums with their discoveries. The ranchers have hit no jackpots.

But from the Indians to the Freemen this is a thin history. This is not a depositional environment, for human culture or rock. The badlands erode and the people leave. The past remains. About a million acres of Garfield County are occupied by the Charles M. Russell National Wildlife Refuge. The rest is a mix of public land overseen by the federal Bureau of Land Management and private ranches. All of it is badlands, crisscrossed with a grid of gravel roads that organize what would otherwise feel truly desolate. The landscape can be beautiful when deep shadow and blinding light fragment the geometry of the gullies and bluffs. It is raw and unpolished, by people or nature.

Leaving Jordan for fossil hunting, you take gravel, or plain dirt roads. You establish a camp, with all the amenities of the modern age that you can muster. Electronics are easier than plumbing. You can set up a satellite dish for broadband computer access. You can even webcast from camp. But outhouses are the rule. When you leave camp for the day to prospect, or dig, you leave the outhouses behind.

Once you are in the field and you are excavating the thighbone of a tyrannosaur, you have gone millions of years back in time. You stand, with parched throat and sweat-soaked shirt, without shade or water, in the present. But you are really back in the Cretaceous, as the fossils and geology show, in a marshy river delta on the shores of a shallow sea that bisected the North American continent from the Arctic to the Gulf of Mexico.

And you can see all this in the rocks. This readability of the earth’s past is a wonder many of us take for granted, speaking of the Cretaceous or the Jurassic, offhandedly describing the animals that lived then, the environment, the weather, temperature, and the arrangement of the continents. But our ability to look back at the past, to re-create it with some confidence, at least in broad strokes, is a wonder that far surpasses the tales told by any religion.

We are now multiplying this wonder, adding the tales told by molecular fossils, and the history of life’s evolution written in the DNA of living animals. What we find gives us new understanding of the past, and new ways to try to reconstruct it. And some of the most astonishing finds, as has been true for at least a century, came from the Hell Creek Formation in Garfield County.

To understand the place of the Hell Creek Formation in the history of life on earth it is necessary to step back a bit, perhaps not to the origin of the planet four and a half billion years ago, but at least to the beginning of the dinosaurs’ reign. By the time of the nonavian dinosaurs’ extinction, this extraordinary group of animals had already had quite a successful run.

All of life’s history is of a piece, so it is awkward to step in at any given point. At the time of the origin of dinosaurs 225 million years ago, during the Triassic period, the big stories in the evolution of life were finished. The first hints of life appeared about 3.8 billion years before the present, and the first, most primitive soft-bodied animals not until about 650 million years ago.

This was just before an enormous blooming of animal forms in the Cambrian period that filled the seas with wriggling, swimming, voracious life. Vertebrates evolved. And fish. About 360 million years ago animals and insects began to colonize the land. That was when the first tetrapod appeared. Tetrapods, four-limbed vertebrates, are, or should be, near and dear to us, since we ourselves are tetrapods. We are one variation on the tetrapod theme that has been sung by natural selection for several hundred million years. We may not be able to claim the antiquity of jellyfish, the variety of insects, the biomass of bacteria. But still, we have reason to take some satisfaction in how successful this basic body plan has been. Reptiles are one highly successful variation, and among the reptiles the much-beloved dinosaurs are unarguably memorable.

We cannot point to a particular fossil and say, here, this is the ancestor of the dinosaurs. In fact, evolutionary biologists have changed how they group animals and categorize descent from ancestral forms, since the familiar and easily understood tree-of-life diagram was developed to grace textbooks and magazine articles. Biologists no longer look for “the” ancestor. Instead they concentrate on shared characteristics that define one group and the new characteristics that appear in the course of evolution to define a new group.

These new traits are called derived characteristics. And the groups are called clades. A diagram of the course of evolution using clades is a cladogram and it is somewhat similar to the old tree. Single-celled organisms are at the start of things, fish appear before amphibians, and both before reptiles. Humans, of course, branch out very late from our mammalian and primate origins.

But cladograms don’t pin down ancestry to one species or genus. A cladogram shows, for example, that from the vertebrates new clades have evolved that have all the shared characteristics of vertebrates plus some new, derived characteristics. For instance, all vertebrates have a backbone. A clade that evolved from the vertebrates, like the mammals, shares the backbone and vertebrate body plan, with eyes and mouth at the front and a digestive system that goes from front to back, among other details.

But the mammals have derived characteristics like mammary glands and fur that we don’t share with other vertebrates. By looking at more detailed skeletal characters, we can see that the ancestors of the mammals branched off from the reptiles before the evolution of the dinosaurs. In fact, the mammals and the dinosaurs appeared around the same time.

Scientists agree that all the dinosaurs come from one ancestral source. The dinosaurs themselves are in two groups, the Ornithischia and Saurischia, based on the structure of their hips, but these groups have a common dinosaur ancestor. We can’t say what that specific ancestor is, or what creature immediately preceded the dinosaurs, but we do think it was something like
Lagosuchus,
a reptile that was less than a meter long and walked on its two hind legs. The first dinosaurs were also bipedal, and the four-legged stance of familiar animals like
Triceratops
and
Brontosaurus (Apatosaurus)
evolved later.

A mass extinction killed the dinosaurs and it may have been a mass extinction that gave them their start. There were two extinctions in the Triassic, one around 245 million years ago, perhaps caused by an asteroid hitting the earth, and one 205 million years ago. After the first, many of the ancestors of mammals and dinosaurs disappeared, leaving some good opportunities. The dinosaurs took over. The mammals stayed in the background for 145 million years until the next mass extinction did away with the dinosaurs and once again offered abundant unfilled ecological niches.

The time of the dinosaurs’ origin was a good one for land animals in the sense that all the modern-day continents were united into one landmass called Pangaea. These continents had previously shifted and drifted apart in various combinations, and once before had united in a supercontinent called Gondwanaland. That broke up, and so, eventually, did Pangaea. The continents continued to drift as the dinosaurs evolved.

Dinosaur species appeared and disappeared over the next 140 million years, taking on the numerous forms that fossils have preserved, the gigantic sauropods, like
Apatosaurus,
carnivores like
Allosaurus
and
T. rex,
the plant eating duck-billed dinosaurs, small colonial nesters such as
Protoceratops,
agile small hunters like
Deinonychus
and
Velociraptor
. Along the way, birds emerged.

By the time of the latest Cretaceous, the period preserved in the Hell Creek rocks, Antarctica, Australia, and South America had separated from the unified landmass and were not connected to other continents. But Africa was still attached to Europe and northern land bridges connected North America, Europe, and Asia.

The sauropods were gone, and among the dominant land animals was
Tyrannosaurus rex
. Duck-billed dinosaurs abounded, as did
Triceratops
and other dinosaurs like the
Pachycephalosaurus
. The late Cretaceous was a time of mountain building in North America. The rising mountains that we now call the Rockies were being eroded as fast as they grew and were drained by rivers and streams that dumped sediment on the plains during floods, burying and preserving dinosaur bones.

The rivers and streams in what is now Montana flowed into the inland sea, which expanded and contracted over millions of years. By the time of the latest Cretaceous, when the Hell Creek sediments were deposited, the sea had retreated and the section of the formation in Garfield County, from which paleontologists have drawn so many fossils, was a river delta, with winding channels, and both land and water habitats, near the coast of the inland sea.

The vegetation was thick and varied, including ferns, conifers, and flowering trees. These are known both from fossils and the microscopic analysis of pollen grains in the rock. Herds of dinosaurs fed on the lush plant life and were preyed on by packs of smaller, hunting dinosaurs like
Troodon.
Lizardlike predatory mosasaurs and long-necked plesiosaurs swam in the inland sea. Mollusks were present in the seas, as well as ponds and rivers, which played host to fish, amphibians, crocodilians, and turtles; all of these survived and continued to thrive when the dinosaurs disappeared. There were land, shore, and diving birds and numerous mammals, primitive relatives of today’s egg-laying platypus and echidna, marsupials, and placental mammals. Some of these would have run up and down the trees like squirrels. Others would have lived on the ground, perhaps in burrows.

One day, while life hummed, chewed, killed, and died, as usual in this tropical environment, a meteor ten to fifteen kilometers in diameter—six to nine miles—entered the earth’s atmosphere, headed for the sea near the Yucatán Peninsula at about seventy thousand miles per hour (thirty-two thousand meters per second). It was traveling at a thirty-degree angle when it struck. The meteorite vaporized, as did its target. One estimate is that twelve thousand cubic miles of debris were sent into the atmosphere. And the energy released has been estimated at one hundred million megatons. The blast at Hiroshima is estimated at fifteen kilotons. In other words, the explosive energy released when the meteorite hit the earth was the equivalent of 6.6 billion atomic bombs like the one dropped on Hiroshima exploding simultaneously.

The result was a worldwide disaster, although it is hard to pin down the exact effects. The impact coincided with a mass extinction that wiped out 35 percent of the species on earth, including all of the nonavian dinosaurs. For a brief time the foundation of the world ocean ecology, the community of microorganisms that harvest the energy in sunlight, was devastated.